Oil that has flowed from an engine to an oil pan is forced to flow back to the engine by an oil pump, for example. In a flow passage (oil passage) in which oil circulates, a valve for controlling the flow rate of the oil may be used. One of the known valves is a thermo valve that operates in accordance with the temperature of the oil. A technique disclosed in Japanese Patent Application Laid-Open Publication (Kokai) No. 2016-27253 is an existing technique pertaining to thermo valves.
An oil flow passage in an engine such as the one described in Japanese Patent Application Laid-Open Publication No. 2016-27253 includes an engine, an oil pan to which oil that has passed through the engine flows, an oil pump for circulating the oil that has accumulated in the oil pan, a relief valve and a thermo valve for controlling the flow rate of the oil circulated by the oil pump, a main flow passage that connects the engine, the oil pan, and the oil pump and allows the oil to circulate therein, and a bypass flow passage that bypasses the main flow passage.
The thermo valve includes a thermo actuator that operates in accordance with the temperature of the oil, and the thermo actuator actuates the valve body. When the temperature of the oil is high, wax inside the thermo actuator expands to move the valve body forward. The valve body closes the bypass flow passage, and the oil flows only in the main flow passage. When the temperature of the oil is low, the wax inside the thermo actuator contracts. The valve body is retracted by the biasing force of a return spring inside the thermo actuator. Thus, the bypass flow passage opens. The oil flows in both the main flow passage and the bypass flow passage.
Oil is highly viscous at a low temperature. In other words, low-temperature oil has high oil pressure. If a portion of the low-temperature oil flows into the bypass flow passage, it becomes possible to keep the oil pressure inside the main flow passage to stay substantially constant regardless of the difference in the oil temperature.
There is a demand that components such as an engine and an oil pan be contained in a small compartment space. When the size of a thermo valve can be reduced, the degree of freedom in the arrangement of the components increases, and this is preferable.